1. Field of the Invention
The present invention relates to processing of crystalline materials, particularly sapphire.
2. Description of the Related Art
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as alumina or silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce an ingot, such as a sapphire ingot, by the HEM method, solid feedstock, such as alumina, is provided in a crucible containing a monocrystalline seed (which comprises the same material as the feedstock but with a single crystal orientation throughout) placed into the hot zone of a solidification furnace. A heat exchanger, such as a helium-cooled heat exchanger, is positioned in thermal communication with the crucible bottom and with the monocrystalline seed. The feedstock is then heated to form a liquid feedstock melt, without substantially melting the monocrystalline seed, and heat is then removed from the melted feedstock by applying a temperature gradient in the hot zone in order to directionally solidify the melt from the unmelted seed. By controlling how the melt solidifies, a crystalline material having a crystal orientation corresponding to that of the monocrystalline seed, and having greater purity than the starting feedstock material, can be achieved.
The sapphire material produced in a crystal growth furnace, often called a boule, takes the shape of the crucible that was used. Typically, sapphire is crystallized in a crucible having a circular cross-sectional shape since this geometry generally results in a more consistent temperature distribution. After growth is complete, the boule is then removed from the crucible and further processed, such as cutting, slicing sawing, grinding, or polishing, to provide sapphire products needed for a variety of applications, such as wafers used as a substrate in several types of electronic devices.
However, as is known in the art, sapphire includes one of several different crystalline axes, such as the c-axis, the m-axis, or the a-axis, and the properties of sapphire vary depending on this crystal orientation. Identifying the proper crystal orientation for a particular application and aligning the sapphire boule to be processed in the proper direction relative to that orientation is both difficult and time consuming, particularly for a boule having a circular cross-sectional shape. Processes are known in which a sapphire boule is placed on a platform and an x-ray module (comprising an x-ray source and detector) positioned at the appropriate Bragg angle for a desired crystal orientation, is used to measure the orientation of the crystal relative to the processing direction. However, such processes are a 2-dimentional analysis, identifying only one crystal plane and orienting the boule along this plane for processing. No additional crystal orientations are considered, and these are often at least as critical to producing sapphire parts having optimized properties. Additionally, proper 3-dimensional orientation cannot be verified, particularly since the shape of a sapphire boule does not necessarily align with the planar axes.
Therefore, there is a need in the industry for improved processes and apparatuses for identifying and aligning a crystalline material, such as a sapphire boule, relative to a processing direction in all three dimensions.